High-Speed Magnets: Exploring Faraday's Law and Lenz's Law *
|Time Required||Very Short (≤ 1 day)|
|Prerequisites||This project requires a high-school level understanding of electromagnetism, or a willingness to learn about it.|
|Material Availability||A kit containing the materials for this project is available from our partner Home Science Tools.|
|Cost||Average ($40 - $80)|
|Safety||Neodymium magnets must be handled carefully. Please see the detailed warnings in the Procedure.|
Safety Notes about Neodymium Magnets:
- Handle magnets carefully. Neodymium magnets (used in this science project) are strongly attracted and snap together quickly. Keep fingers and other body parts clear to avoid getting severely pinched.
- Keep magnets away from electronics. The strong magnetic fields of neodymium magnets can erase magnetic media like credit cards, magnetic I.D. cards, and video tapes. It can also damage electronics like TVs, VCRs, computer monitors, and other CRT displays.
- Keep magnets away from young children and pets. These small magnets pose a choking hazard and can cause internal damage if swallowed.
- Avoid use around people with pacemakers. The strong magnetic field of neodymium magnets can disrupt the operation of pacemakers and similar medical devices. Never use neodymium magnets near persons with these devices.
- Use the magnets gently. Neodymium magnets are more brittle than other types of magnets and can crack or chip. Do not try to machine (cut) them. To reduce the chance of chipping, avoid slamming them together. Eye protection should be worn if you are snapping them together at high speeds, as small shards may be launched at high speeds. Do not burn them; burning will create toxic fumes.
- Be patient when separating the magnets. If you need to separate neodymium magnets, they can usually be separated by hand, one at a time, by sliding the end magnet off the stack. If you cannot separate them this way, try using the edge of a table or a countertop. Place the magnets on a tabletop with one of the magnets hanging over the edge. Then, using your body weight, hold the stack of magnets on the table and push down with the palm of your hand on the magnet hanging over the edge. With a little work and practice, you should be able to slide the magnets apart. Just be careful that they do not snap back together, pinching you, once you have separated them.
- Wear eye protection. Neodymium magnets are brittle and may crack or shatter if they slam together, possibly launching magnet fragments at high speeds.
- Examine the equation form of Faraday's law, which states that the voltage generated in a coil of wire is proportional to the number of turns in the coil, and the rate of change of the magnetic flux. How can you manipulate either one of these two variables and measure the resulting change in voltage? Here are a few hints:
- The rate of change of magnetic flux depends on how fast the magnets are moving through the tube. Rather than shaking the generator (which would make the magnets' velocity difficult to measure), construct a longer tube and drop the magnets through it from a known height, which should allow you to calculate their velocity when they pass through the coil.
- In order to properly measure the voltage, you will need to use a multimeter set to measure AC voltage, or an oscilloscope. If you do not have access to either tool, you can use LEDs in series as a very rough measurement of the peak voltage (as described in the original Human-Powered Energy project idea). Keep in mind that voltages add in series, and the forward voltage drop of a red LED is roughly 2.2 V.
- Lenz's law describes how the polarity of the voltage induced in a coil depends on the direction a magnet's poles are facing and the direction in which the magnet is moving. Connect two LEDs to your coil with opposite polarities, meaning the LEDs should be wired in parallel, but facing opposite directions—the anode (long leg) of one LED should be connected to the cathode (short leg) of the other LED, and vice versa. How can you use this configuration to demonstrate Lenz's law? When you shake the generator, will the LEDs flash at the same time, or will they alternate? Here are a few hints:
- The timing of the LED flashes may be difficult to see with the naked eye. Many modern smartphones and digital cameras have a "slow motion" video capture option, which will record video at 60 or even 120 frames per second. Use this ability to film the LEDs as you shake the generator. Use a media player that lets you step through a video frame-by-frame to closely examine the timing of the LED flashes.
- Construct the tube for your wire coil using a transparency sheet (available at Amazon.com or an office supply store) instead of cardstock. This will allow you to see the magnet moving inside the coil. Rather than shaking the generator with your hand, which would be difficult to film, set up a longer tube and drop the magnet through it from a known height. Set up your camera so you can film the magnet falling through the tube and see the LEDs flashing in the same frame, as shown in Figure 1. This will allow you to correlate the position and velocity of the magnet within the tube with the timing and brightness of the LED flashes. Do your findings match up with your predictions based on Lenz's law?
Figure 1. This experimental setup has two LEDs connected with opposite polarity, and a generator with a clear tube so you can see the magnets moving inside.
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Last edit date: 2019-01-16
To learn more about magnetic induction, Faraday's law, and Lenz's law, see the following website, or consult your high school physics textbook:
- Nave, C. (n.d.). Faraday's Law. Hyperphysics. Retrieved December 4, 2014, from http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html
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- Transparency sheets, available from Amazon.com
- Corrugated cardboard
- Craft knife or utility knife
- Super glue
- Smartphone or digital camera with slow-motion video capture mode
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In your email, please follow these instructions:
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- Please describe how you need help as thoroughly as possible:
Good Question I'm trying to do Experimental Procedure step #5, "Scrape the insulation from the wire. . ." How do I know when I've scraped enough?
Good Question I'm at Experimental Procedure step #7, "Move the magnet back and forth . . ." and the LED is not lighting up.
Bad Question I don't understand the instructions. Help!
Good Question I am purchasing my materials. Can I substitute a 1N34 diode for the 1N25 diode called for in the material list?
Bad Question Can I use a different part?
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